Method for heat treating wire or the like

ABSTRACT

VARIOUS ARRANGMENTS AND METHODS ARE DESCRIBED FOR ELECTRICALLY HEATING A RAPIDLY TRAVELING WIRE AS IT PROCEEDS ALONG A HELICAL PATH AROUND A GUIDE ROLL. A PRESSURE ROLL STRUCTURE COOPERATES WITH THE GUIDE ROLL TO MAINTAIN ADJACENT TURNS OF THE HELIX IN INTIMATE CONTACT WHERE REQUIRED OR IN CONTACT WITH ITSELF AND THE GUIDE ROLL. ONE, OR BOTH, OR NEITHER, OF THE ROLL MEMBERS MAY HAVE AT LEAST AN ELECTRICALLY CONDUCTIVE SURFACE FOR PROVIDING A PATH FOR BRIDGING CURRENTS BETWEEN THE HELICAL TURNS OF THE WIRE. THE PRESSURE ROLL STURCTURE MAY BE SEGMENTED AND FLOATINGLY MOUNTED. THE INDUCTION COIL MAY BE NON-UNIFORM IN DIAMETER TO PROVIDE CONTROL OF THE HEAT GENERATED OVER DIFFERENT PORTIONS OF THE WIRE HELIX.

A rils, 1971' -RP 3,514,005

METHOD FOR HEAT TREATI N G WIRE OR THE LIKE Filed June 12, 1968 2sheets-sheet 1 INVENTOR MAMCF C. F000 W s! zama 06 April 6, 1971 w. c.RUDD 3,514,005

METHOD FOR HEAT TREATING WIRE OR THE LIKE Filed Jun 12, 1968 2Sheets-Sheet 2 9 INVENI'OR. MMuc: C? Fun) 3,574,005 METHOD FOR HEATTREATING WIRE OR THE LIKE Wallace C. Rudd, Larchmont, N.Y., assignor toAMF Incorporated, New York, N.Y. Filed June 12, 1968, Ser. No. 736,415Int. Cl. C21d 1/42 US. Cl. 148-150 4 Claims ABSTRACT OF THE DISCLOSUREVarious arrangments and methods are described for electrically heating arapidly traveling wire as it proceeds along a helical path around aguide roll. A pressure roll structure cooperates with the guide roll tomaintain adjacent turns of the helix in intimate contact where requiredor in contact with itself and the guide roll. One, or both, or neither,of the roll members may have at least an electrically conductive surfacefor providing a path for bridging currents between the helical turns ofthe wire. The pressure roll structure may be segmented and floatinglymounted. The induction coil may be non-uniform in diameter to providecontrol of the heat generated over different portions of the wire helix.

The present invention relates to high speed heat treatment of metal incontinuous form. More specifically, it relates to apparatus and methodsfor heat treating wire or metal ribbon for such purposes as forming,annealing or hardening.

In general, it is desirable that the heat treatment be performed whilethe wire or ribbon is being transported at high speed from a source ofsupply to a take-up or utilization device. While this might not seem topresent a problem, the contrary is actually the case.

Considerable thermal energy must be generated in the metal to producethe desired temperature. Usually, space is limited and it becomesnecessary to produce such thermal energy within the confines of suchspace restriction.

All materials change physical dimension when heated, and as thetemperature of wire or metal ribbon is raised it will expandsubstantially in the longitudinal direction. At the same time, it losestensile strength. Slack that is developed by the expansion must be takenup without unduly straining the softened material.

Where heat is produced by the passage of electric currents through zonesof the wire or ribbon, cognizance must also be taken of the change inelectrical resistance that occurs as the temperature increases, and ofits effect upon the production of thermal energy. This is particularlysignificant and can pose a major problem in the heat treatment of ironor magnetic steel. When these materials are heated above the Curie pointthe slope of the resistivity versus temperature characteristic changesabruptly.

The problem is further complicated when it is desired to use highfrequency current for electric heating. Skin effects or penetrationphenomena are encountered. Changes in resistance are now even moresignificant. In addition, the relative permeability of the metal becomesa significant factor and, as is well known, the permeability of steeldecreases abruptly when its temperature exceeds its Curie point.

The penetration problem in heat treating metal with high frequencycurrents and its control is discussed in detail in Letters Pat. No.3,037,105 issued May 29, 1962 on an application of Fred Kohler andassigned to the same assignee as the present application. In summary, itcan be stated that when a current of high frequency is United StatesPatent 3,574,005 Patented Apr. 6, 1971 caused to flow in a conductor ofcomparatively large cross-section, the conductor appears to present moreresistance to its flow than it does to a direct current. The increase isdue to E.M.F.s which are set up by variations of magnetic flux withinthe conductor itself. These crowd the current toward the surface. Thedepth of penetration of the current in the conductor is a directfunction of the resistivity of the conductor and an inverse function ofboth the relative permeability and the frequency.

Mention has been made separately of the change in resistance withtemperature and the change in permeability with temperature. It can alsobe appreciated that an interrelationship exists which furthercomplicates the problem. Thus, consider the application of electrodes toa magnetic steel wire. Assume that an alternating current source iscoupled to the electrodes to cause current to flow through the wirebetween the electrodes and that the frequency is high enough to producea significant skin effect. The current will be crowded towards thesurface of the wire and the effective resistance as seen by theelectrodes and source will be greater than the DO resistance.

Heat will be produced in the wire in accordance with the well known 1 Rlaw. But as the wire temperature increases so does the resistance. Allother factors remaining constant, this would result in a proportionaldecrease in current (it is assumed that the source voltage remainsconstant) and a decrease in the amount of heat generated. However, theresistance affects the penetration depth with the depth increasing as afunction of the square root of the resistance, or more accurately theresistivity. The current distributes through a greater volume of thewire and the apparent resistance is thereby altered with its concomitanteffect upon the current and so forth. Fortunately, below the Curie pointthe variation with temperature tends to be gradual. But when the Curiepoint temperature is exceeded there is an abrupt change in relativepermeability causing an abrupt change in penetration depth with itsassociated change in effective resistance, change in current flow, andchange in heat generation. Add to this the abrupt change in slope of theresistance characteristic and it can be seen that extremely precisecontrol is necessary to provide any degree of control over the heattreatment given to the wire.

Because it avoids contact problems, electrical induction heating isoften more desirable than electrical conduction heating. However, if anattempt is made to heat a small piece of wire by passing the wirecoaxially through an induction coil, difficult loading problems areencountered. It is difficult to induce a large amount of power into theWire because of the limited depth of penetration of the current. Forexample, at 10,000 c.p.s. the wire diameter must exceed /2 inch beforeany appreciable power can be induced in it. Even at a frequency of 500KC severe problems are encountered with diameters less than 4 inch.

It has been suggested, heretofore, that wire can be heat treated bywinding it on and off a mandrel to form a helix thereon and inducingelectric current therein from a primary coil associated with themandrel. As the mandrel is rotated, the wire is wound on one end and offthe other end. However, the method has certain problems associated withit.

For the mandrel and helix method to merely function some conductiveconnection must be established between adjacent turns of the helix topermit the currents to bridge or jump from turn to turn at some point incircling the helix concentric with its axis. If the method is to be usedfor controlled heating of wire traveling over the mandrel at high speed,not only must contact be established between adjacent turns but suchcontact must be maintained uniform from turn to turn. Contactresistances must remain as constant as possible and should not vary fromcontact to contact. The length of turn between adjacentturn contactshould be controlled precisely. If uniform contact is not maintained,arcing may develop, burning the wire and interfering with uniform andcontrolled heat generation. Variation in contact resistance, either withor without arcing, alters the load on the power source with concomitantcurrent change and heat variation. Therefore it should be evident thatbefore this method can be put to practical use any tendency of the turnsto separate, due to the flow of currents in the same direction inside-by-side turns (a significant phenomenon with heavy gauge wire) mustbe counteracted. Alternatively, the need for actual engagement betweenadjacent turns must be eliminated. Similarly, as the wire expands due toheating, the slack must be absorbed without applying sufiicient force tothe wire to deform it in its softened state, all with a view towardmaintaining uniform electrical connection between the turns.

It is, therefore, an object of the present invention to provideapparatus and methods for overcoming the disadvantages enumerated abovein the above described type of rapid heat treatment of wire or the like.

In accordance with an aspect of the invention there is provided a methodof heat treating wire or the like which comprises the steps of feedingthe wire to be treated into one end of a nip formed between a pressureroll and a guide roll mounted for rotation about substantially parallelaxes, conveying the wire around a helical path passing through the niponce each turn then away from the nip near the other end thereof after apredetermined number of turns, moving the wire from the one end towardthe other end of the nip while it traverses the helical path asadditional wire is fed into the nip at the one end, pressing the rollstoward each other to confine the wire in the nip to establish andmaintain uniform electrically conductive connections between adjacentturns at least in the region of the nip, and supplying high frequencyelectrical heating current to the wire while the wire traverses thehelical path.

The invention will be better understood after reading the followingdetailed description of various embodiments thereof with reference tothe appended drawings in which:

FIG. 1 is a perspective diagrammatic view illustrating one preferredform of the apparatus;

FIG. 2 is a sectional view taken along line 2-2 in (FIG. 1;

FIG. 3 is a perspective view of the induction coil employed in theapparatus of FIGS. 1 and 2;

FIG. 4 is a fragmentary view similar to FIG. 2, showing a modificationof the pressure roll therein;

FIG. 5 is a sectional view taken along the line 5--5 in FIG. 4;

FIG. 6 is a view similar to FIG. 4 showing a further modification of theinvention;

FIG. 7 is an end view of apparatus similar to that of FIG. 1 showing amodified form of the induction coil;

FIG. 8 is a view similar to FIG. 2 showing a further modification of theinvention;

FIG. 9 is an end view of the structure in FIG. 8;

FIG. 10 is a view similar to FIG. 8 showing a still further embodimentof the invention; and

FIG. 11 is an end View of the structure in FIG. 10.

Throughout the drawings the same reference numerals will be employed todesignate the same or similar part.

Reference should now be had to FIGS. 1 to 3. There is shown therein anarrangement requiring comparatively little space for heating wire whileit is traveling at a very high rate of speed. The wire to be heattreated is designated generally by the numeral 20 and is fed at 21 ontoa metal drum 22 mounted for rotation about a shaft or axis 23. Aftermaking a number of turns about the drum 22, the end of the wire, 24, iswithdrawn by known means not shown. As the end 24 of the wire iswithdrawn, the drum 22 rotates, and the turns of wire 25 work themselvesacross the drum in screw" fashion. A reaction roll 26, best seen in FIG.2, provides a reaction surface forcing the wire as it winds upon thedrum to move away from the roll 26 toward the opposite end of the drum22. The reaction roll 26 may be mounted for rotation about a shaft 27.

In order to ensure electric connection between adjacent turns of thewire while on the drum, there is provided a pressure roll 28 mounted forrotation about an axis or shaft 29 mounted parallel to the shaft 23 ofthe drum 22. As best seen in FIG. 2, a nip 30 is formed between thepressure roll 28 and the metal drum 22. The metal drum may be consideredas a guide roll for the Wire. The pressure roll 28 may be formed fromeither hard or soft material, conductive or insulating, depending uponthe working temperature of the wire being treated and the currentpenetration and heat generation in the pressure roll itself.

In order to supply current to the helical section of the wire beingtreated, there is provided a single turn induction coil 31 having endterminals 32 and 33 to which a source of high frequency current 34 isconnected, as shown. In the embodiment now being described withreference to FIGS. 1, 2 and 3, it is assumed that the pressure roll 28is electrically conductive. In order to avoid inducing heat generatingcurrent in the electrically conductive pressure roll 28, the inductioncoil 31 is provided with an aperture or slot 35 permitting the pressureroll to engage the wire being treated while remaining essentiallyoutside of the magnetic field created by the induction coil.

By making the drum 22 reasonably large in diameter it is possible toinduce very large currents into the helical turns 25 of the wire beingtreated. The generator 34 may operate at frequencies ranging from about960 cycles to 500,000 cycles per second for best results. If asufficiently large drum can be employed, current having a frequency aslow as 60 cycles may be employed. Essentially, the frequency should bechosen so that the depth of penetration of the current, as explainedabove, is such that almost all of the current flows in the thickness ofthe wire layer around the drum 22 and very little current flows in thedrum itself. The drum should be arranged to conduct current only to theextent necessary to ensure electric cross-over between adjacent turns.

To illustrate the effect of frequency, for a inch diameter steel wireheated above the Curie point, the depth of current penetration would beabout inch at 10,000 cycles. Therefore, 10,000 cycles can be usedsatisfactorily. As another example, steel wire of 0.040 inch diametercan be heated above the Curie point using 500,000 cycles as thefrequency of the source since the depth of penetration above the Curiepoint at this frequency is about 0.030 inch.

When the drum 22 is conductive, the induced current in the wire beingtreated crosses from turn to turn by contact both between the turns andthrough the metal drum. In addiion, if the depth of penetration of thehigh frequency current is greater than the diameter of the wire, somecurrent is induced directly in the drum. Under certain conditions, theflow of current in the drum may prove a problem and can be avoided bymaking the drum of insulating material such as porcelain, Steatite, orPyrex glass. In this case, the induced current is restricted to flowonly in the wire layer. However, by virtue of the restraint provided bythe pressure roll 28, either or both of the following conditions may beestablished to provide for uniform cross-over current: (a) adjacentturns of the wire are maintained in intimate contact; (b) the crossovercurrent is caused to flow through the conductive surface layer of thepressure roll where it engages the wire.

A unitary pressure roll such as the roll 28 in FIG. 1 may not be able toprovide the necessary restraint for or contact with the turns of wire ifthe diameter of the wire should vary or for other reasons. Under suchcircumstances, it may be preferred to employ a pressure roll constructedas shown in FIG. 4. As seen therein, the pressure roll is subdividedinto a plurality of individual adjacent roll segments, such as segments36 and 37, arranged for independent rotation. Each segment has an axiallength equal substantially to twice the diameter of the wire 38. As seenin FIG. 5, the segments of the pressure roll of FIG. 4 are mounted on aunitary shaft 39 with each roll segment comprising a radial outerelement 40 and an inner element 41 separated by an anti-friction bearing42. A layer of elastomeric material 43 is disposed between the shaft 39and the inner element 41. By virtue of this construction, the individualroll segments are able to float and adapt to variations in wirediameter. A further advantage of the individual roll segments is thatthe wire can slip around faster without frictional resistance as itlengthens with heat and progresses along the drum because each rollsegment can turn independently.

The arrangement shown with reference to FIGS. 4 and 5 is well suited tothe handling of small diameter wire, say, for example, less than /8 inchin diameter. Where larger diameter wire is to be heat treated, it willbe found advantageous to employ the modification shown in FIG. 6. Asshown therein, there is a separate independent roll segment, such as thesegment 44, for each passage of the wire through the nip. The axialextent of each roll segment is equal to the diameter of the wire. Inaddition, each roll segment is provided with a peripheral groove forreceiving and guiding the wire. In all other respects, the segments ofthe pressure roll in FIG. 6 may be identical to those shown in FIGS. 4and 5. That is, the segments may be resiliently mounted for frictionfree rotation and floating action.

It is to be understood that the modifications described above withreference to FIGS. 4, 5 and 6 relate to the construction of the pressureroll. The remainder of the apparatus may be the same as that shown inFIGS. 1, 2 and 3. On the other hand, where the pressure roll isnonconductive to electricity, it is possible to employ an alternativeinduction coil construction as shown in FIG. 7. As shown therein, thecoil 45 encircles in close proximity the guide roll or drum 22 with thewire wrapped therearound. In addition, the coil 45 extends over andaround in close proximity to the pressure roll 28. The roll 28 is usedmerely as an example since any of the modified roll constructions can beemployed.

The various embodiments described to this point employ a pressure rollhaving a length substantially equal to the length of the drum or guideroll. In addition, the width of the induction coil approximates thewidth of the helix formed by the wire wrapped upon the drum.Consequently, during operation of the device, heat is generated withinthe entire region of the helix. There are instances, however, where itis desirable to place a utilization device immediately after the heatingstation. An example is the situation where the wire is annealed immediately prior to drawing. Since the space between the drawing dies islimited, the heating and cooling steps of annealing must be compressedinto a restricted area. This can be handled readily with theconstruction shown in FIGS. 8 and 9.

As best seen in FIG. 8, the pressure roll 46 is shorter than the guideroll 22 developing a nip 47 along only a part of the length of the guideroll. The helical path of the wire, however, extends both throughout thenip 47 and for a substantial distance beyond on the guide roll 22. Theinduction coil 48, here shown as extending around both the guide rolland the pressure roll, has an axial length substantially equal to thenip 47. Thus, the wire is heated while it traverses the helical pathcoinciding with the nip 47 and cools while it traverses the remainder 6of the helical path. Cooling of the wire may be accomplished either byradiation to the atmosphere or by positive cooling with aquenchinginedium. 1

When wire is heated by the apparatus described herein, the resistance ofthe wire will increase with tempera ture. In addition, the wire willradiate more thermal energy as its temperature increases and, of course,with magnetic steel as noted above there exists the Curie point problem.Therefore, circumstances may arise requiring that power be introduced tothe wire at varying rates from one end to the other end of the helix.For this purpose, the induction coil may be tapered as shown by the coil49 in FIGS. 10 and 11. By making the diameter of the coil 49 smaller atthe take-off end of the drum where the wire is hottest, maximum energycan be induced at this end with lesser energy induced at the point ofentry. This arrangement is useful wherever the rate of temperature risemust be controlled for radiation, metallurgical, or other reasons. Asshown in FIGS. 10 and 11, the coil is provided with an aperture 50through which the pressure roll, for example the roll 28, may extend. Asbest seen in FIG. 11, the larger diameter end of the coil which bypassesthe end of the pressure roll is provided with a dip to clear the rollshaft 29. Of course, if the pressure roll 28 is made of non-conductivematerial, the coil may extend around the roll as shown in FIG. 7.

What is claimed is:

1. The method of heat treating wire or the like comprising the steps offeeding the wire to be treated into one end of a nip formed between apressure roll and a guide roll mounted for rotation about substantiallyparallel axes, conveying the wire around a helical path, the wirepassing through said nip once each turn then away from said nip near theother end thereof after a predetermined number of turns, moving saidwire from said one end toward said other end of the nip while ittraverses said helical path as additional wire is fed into said nip atsaid one end, pressing said rolls toward each other to confine said wirein the nip to establish and maintain uniform electrically conductiveconnections between adjacent turns at least in the region of said nip,and supplying high frequency electrical heating current to said wirewhile the wire traverses said helical path.

2. The method according to claim 1, wherein said high frequency currentis supplied to said wire by means comprising an induction coilsubstantially encircling but spaced from said guide roll, and whereinsaid wire is fed to the nip so as to provide a substantially continuouslayer of abutting turns intermediate the face of said guide rollengaging said turns and substantially all of the face of said coilnearest said guide roll, and wherein the frequency of said current isselected so as to provide a penetration depth in the metal of said =wiresubstantially equal to the thickness of said wire.

3. The method according to claim 1, wherein said high frequency currentis supplied to said wire with varying power at different points alongsaid helical path for controlling the heating therealong.

4. The method according to claim 1, wherein the wire is conveyed arounda continuation of the helical path after it leaves the nip, and whereinsaid high frequency current is supplied to the wire bet-ween the time itis fed to and leaves the nip, and wherein the Wire is cooled after itleaves the nip and while it traverses the continuation of the helicalpath.

References Cited UNITED STATES PATENTS 2,040,343 5/1936 Simons et al.148-13 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R.

